Plate-frame heat exchangers are commonly employed to provide relatively compact devices with low-pressure drop. Such devices are typically deployed in weight/volume critical applications such as automotive air-conditioning evaporators, gas turbine recuperators, fuel cells, and liquid—liquid industrial heat exchangers. Because these applications are sensitive to both heat exchanger size and pressure drop through the fluid passages, typical plate-frame heat exchangers have a series of individual heat exchanger cells arrayed substantially in parallel (i.e. each cell (hot fluid side and cold fluid side) has the same temperature distribution as every other cell in the stack of cells comprising a completed heat exchanger)).
Because of the success of the plate-frame approach to heat exchange design, it has been widely adapted to chemical reactions requiring temperature control, especially those that require close temperature control because of product selectivity or are strongly endothermic or exothermic and require rapid heating and cooling.
A common example of considerable importance is steam reforming of hydrocarbons and alcohols (This reaction involves the reversible chemical conversion of methane and water into carbon monoxide and hydrogen). This reaction is highly endothermic, and typically requires large amounts of catalyst to promote the reaction. A compilation of the use of the plate-frame reactors is that the effectiveness in exchanging heat between the cooler reformate stream and the hot combustion products plays a strong role in determining the thermal or thermodynamic efficiency of the reforming system. Effectiveness factor is defined as the temperature that occurs in the fluid undergoing the maximum temperature change divided by the difference between the highest and lowest temperatures in the heat exchanger.
Current technologies have focused on plate-frame reformers having an array of small reactors massively in parallel to each other. This design is far more compact, lighter and less expensive than tubular-type reformers which are common in the industry. However, such reformers have three major drawbacks.
First, massively parallel construction leads to low flow velocity (and corresponding Reynolds number) and low laminar flow. This drawback is critical because lower laminar flow reduces heat transfer rates and reduces reactant mixing in the reactor structures, which along with the Reynolds number are factors in sizing the reformer. Hence, a lower Reynolds number requires a larger reformer, which adds to the cost of the reformer system.
Second, manifolding in the massively parallel construction may be fairly complex. This complexity may cause poor fluid distribution with “dead zones”, where little flow occurs, which reduces heat exchange effectiveness.
Third, controlled internal release of any one reactant is very difficult as the short reaction zone is only accessible from either end of the plate. This point is particularly important if the heat exchanger structure is to be used as a catalytic burner. Catalytic burning on the heat exchanger walls improves heat transfer locally by obviating convective heat transfer from the gas phase to the wall because the catalysts are located on the wall itself. Unfortunately, if fuel or oxidant levels are not controlled, the catalytic burning can occur at too high a rate, causing local increases in metal temperature referred to as hot spots. Hot spots significantly weaken the structure and may cause mechanical failure. Because of this fact, systems with catalytic burning on the wall must use exotic materials and dilute combustion gases to lower temperatures to an acceptable level, which negatively impacts both cost and efficiency.
It is thus highly desirable to design a plate-frame reactor that combats the three critical drawbacks of the massively parallel system.